Electronic devices (typically including one or more integrated circuits) are generally subjected to a test process in order to verify their correct operation; this is of the utmost importance to ensure high quality of a manufacturing cycle of the electronic devices. The test can be aimed at identifying defects that are either evident or potential (i.e., which might occur after a short period of use). At the same time, the integrated circuits under test may be conditioned thermally (so as to ensure that they work at a predetermined temperature). A typical example is the burn-in test, which consists of making the electronic devices work for some hours at very high or very low temperature (for example, ranging from −50° C. to +150° C.); in this way, it is possible to simulate a long period of operation of the same electronic devices at room temperature (i.e., 25° C.-50° C.).
The test may be performed either at the wafer level or at the package level. In the first case, the integrated circuits are tested directly—when they are still included in a wafer of semiconductor material; conversely, in the second case the electronic devices are tested once their manufacture is complete (i.e., the integrated circuits have been diced and embedded in suitable packages). The test at the package level reduces the risks of damages to the integrated circuits (for example, due to atmospheric contaminations or shocks); moreover, it allows testing the electronic devices in their actual final form.
The test of the electronic devices at the package level requires that they should be mounted on test boards, which are used to interface the electronic devices with a test system. For this purpose, each test board is provided with multiple sockets. The sockets lock the electronic devices mechanically and connect them electrically to the test system; at the same time, the sockets allow removing the electronic devices without any substantial damage after they have been tested.
With reference in particular to electronic devices of the Ball Grid Array, or BGA, type (having terminals in the form of small balls), an example of known test board is disclosed in U.S. Pat. No. 6,204,681, which is incorporated by reference. Particularly, the test board has a plurality of sockets each one with a spring lever mechanism to clamp a corresponding electronic device; an opening throughout the test board is formed at the center of the socket to expose the balls of the electronic device. The test board is overturned and pressed against a test head arranged above it. For each socket, the test head includes a set of contact tips for the balls of the corresponding electronic device (which contact tips are mounted on an anisotropic conductive sheet). Each contact tip ends with resilient petals or with a cup-like portion; in both cases, the contact tip has a radius smaller than the one of the ball that is pressed against it, so as to protect its apex, to scrap any oxidation film, and to collect possible debris.
A different socket for electronic devices of the BGA type is described in U.S. Pat. No. 5,808,474, which is incorporated by reference. In this case, the socket is formed by a rigid base with a detachable lid, which is used to lock a corresponding electronic device that has been inserted into the base. The socket is also provided with a bladder formed by a flexible circuit substrate with a plurality of test contacts; when a pressurized fluid is fed to the interior of the bladder, the flexible circuit substrate is raised so that its test contacts are urged against the corresponding balls of the electronic device. Alternatively, the flexible circuit substrate is mounted on a compressible layer, which likewise urges its test contacts against the balls when it is compressed by the electronic device forced downwards by the lid.
A drawback of the test boards known in the art is that the structure of the sockets is relatively complex. Moreover, their assembling on the test boards requires a number of operations; for example, the sockets are soldered on the test boards or they are snap mounted thereon. In any case, this adversely affects the cost of the test boards, and then of the whole test system.
With reference now to the thermal conditioning of the electronic devices under test, the desired result is generally achieved by forcing hot or cold air towards the test boards. However, this requires a very bulky structure that has a detrimental impact on the size of the test system and on its cost. Moreover, the control of the temperature of the electronic devices so obtained is not completely effective. Particularly, it is very difficult (if not impossible) to obtain a uniform distribution of the temperature throughout the different electronic devices.
Different solutions have instead been proposed for testing electronic devices at the wafer level.
For example, U.S. Pat. No. 6,468,098, which is incorporated by reference, mentions a know solution, wherein the wafer to be tested is supported by a rigid membrane (made of steel) that is welded to a bellow; a fluid is passed through the bellow, so as to press the wafer against a chuck structure disposed above it. The fluid may also be heated or cooled to affect a temperature of the wafer. In order to improve the connection to the wafer, this document instead proposes the use of an expandable chamber that is defined by a flexible wiring layer provided with compliant contacts. After mounting the wafer below the flexible wiring layer, a fluid is introduced into the chamber so as to press the flexible wiring layer into contact with the wafer; as above, the fluid may also be used to control the temperature of the wafer. Alternatively, the wafer may be maintained above the flexible wiring layer by a vacuum chuck, which is also used to control its temperature. In another embodiment described in the same document, the wafer is attached above a flexible layer that forms a chamber for a fluid (having a single port, so that no control of the temperature of the fluid can be implemented). When the fluid is introduced into the chamber, the flexible layer is raised so as to push the wafer against a wiring layer that is attached to a chuck.
Another pneumatic system for establishing and maintaining electrical connection between contacts is described in US-A-2004/0056649, which is incorporated by reference. Particularly, this document proposes the use of a bladder that is enclosed within a fixture. When the bladder is pressurized with a fluid, it exerts a force that creates the desired connection between corresponding contact pads (such as to activate a switch). For example, the bladder may act on contact buttons (integral with it or mounted on a planar plate), or on pads with a dendritic interface. The fluid may also be thermally conductive, so as to provide a heat sink for the connections.
Referring back to the test of the electronic devices at the package level, a further drawback of the solutions known in the art is that the corresponding test process is quite complex. Indeed, the electronic devices to be tested are mounted on the test boards in a dedicated assembling station. The test boards are then moved to the test system for testing the (carried) electronic devices. At the end of the test process, the test boards are returned to the assembling station for removing the (tested) electronic devices. It is now possible to insert the electronic devices that passed the test into trays that will be used for their final shipping. However, the above-described operations (and especially the need of assembling/disassembling the test boards and moving them to and from the test system) add further complexity and increase the length of the test process.
All of the above maintains the overall cost of the test process relatively high; this drawback limits the use of the test process, and accordingly lowers the level of quality and reliability in the manufacturing of the electronic devices.